Method and device for ascertaining a temperature of a sheathed-element glow plug in an internal combustion engine

ABSTRACT

In a method for ascertaining a temperature of a sheathed-element glow plug in an internal combustion engine, a temperature difference between the temperature of a glow plug heater inside the sheathed-element glow plug and a temperature at an arbitrary location on the sheathed-element glow plug is determined as a function of at least one operating parameter of the internal combustion engine and/or at least one operating parameter of the sheathed-element glow plug. The temperature of the sheathed-element glow plug is ascertained from a temperature value represented by a measured value, and from the determined temperature difference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for ascertaining a temperatureof a sheathed-element glow plug in an internal combustion engine, inwhich a temperature difference between the temperature of a glow plugheater inside the sheathed-element glow plug and the temperature at anarbitrary location on the sheathed-element glow plug is determined as afunction of at least one operating parameter of the internal combustionengine and/or at least one operating parameter of the sheathed-elementglow plug, and a device for carrying out the method.

2. Description of the Related Art

Sheathed-element glow plugs, which are used in internal combustionengines for igniting a fuel-air mixture, have a heater which preheatsthe cold sheathed-element glow plug to a temperature which is highenough for igniting the fuel-air mixture. However, the distribution ofthe temperature, starting from the heater and extending over the entiresheathed-element glow plug, is very inhomogeneous, resulting intemperature differences between the temperature of the heater inside thesheathed-element glow plug and the temperature at the surface of thesheathed-element glow plug.

Since the sheathed-element glow plug protrudes into the combustionchamber of the internal combustion engine, the surface of thesheathed-element glow plug is always cooled by the air stream whichflows as a fuel-air mixture past the sheathed-element glow plug duringdynamic operation of the internal combustion engine, so that the surfaceof the sheathed-element glow plug is never at the temperature of theheater inside the sheathed-element glow plug.

A method for operating a sheathed-element glow plug is known frompublished European patent document EP 1 719 909 B1, in which thesheathed-element glow plug is situated in the combustion chamber of theinternal combustion engine. The cooling characteristics of thesheathed-element glow plug are computed as a function of the combustiontemperature and the sheathed-element glow plug temperature. Thetemperature of the sheathed-element glow plug is computed from theelectrical power consumed by the sheathed-element glow plug, while thecombustion temperature in the combustion chamber is ascertained as afunction of the air temperature, the cooling water temperature, therotational speed of the internal combustion engine, and the load.

If the temperature of the sheathed-element glow plug is to be subject tocontrol, this control is carried out as a function of the resistance ofa current-conducting wire inside the sheathed-element glow plug, fromwhich the actual controlled value of the temperature is ascertained. Theresistance is higher the greater the temperature of the wire. Due to thetemperature difference which occurs, the quality of the control of thesheathed-element glow plug temperature is inadequate, since this controlis not based on a temperature which is actually present at the surfaceof the sheathed-element glow plug.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and a devicefor ascertaining a temperature of a sheathed-element glow plug in aninternal combustion engine, which allows the temperature differencebetween the temperature of the sheathed-element glow plug at a locationoutside the heater and the temperature at the heater of thesheathed-element glow plug to be determined.

According to the present invention, the object is achieved in that thetemperature difference is determined as a function of a time function,and the temperature of the sheathed-element glow plug is ascertainedfrom a temperature value, which is represented by a measured value, andfrom the temperature difference, which is influenced by the timefunction. The advantage of the present invention is that the temperaturedifference is determined during dynamic operation of the internalcombustion engine, i.e., under the variable operating conditions of theinternal combustion engine. The fact that the fuel-air mixture, whichflows past the sheathed-element glow plug at varying speeds andquantities depending on the operating state of the internal combustionengine, influences the temperature at the surface of thesheathed-element glow plug is taken into account. It is thus possible toprecisely ascertain the temperature of the sheathed-element glow plug innonstationary operation of the sheathed-element glow plug. Applicationsare possible without an additional thermocouple which measures thetemperature at the arbitrary location on the sheathed-element glow plug.

The time function is advantageously determined from a time constantwhich is ascertained once for the sheathed-element glow plug and stored.Thus, only a few values are necessary for determining the temperaturedifference. The time constant indicates the thermal inertia of thesheathed-element glow plug in the respective cylinder of the internalcombustion engine, which is the same for all computation steps.

In another implementation, the time constant is corrected as a functionof a rotational speed of or load on the internal combustion engine in adynamic driving mode or as a function of the voltage, i.e., the power,applied to the sheathed-element glow plug.

The time function also represents a measure of how strongly aninstantaneously determined temperature difference is influenced by apreceding temperature difference.

In one embodiment, the influence on the instantaneously determinedtemperature difference by a preceding temperature difference isdescribed by an exponential function or a power series. The exponentialfunction characterizes in a particularly accurate manner how stronglythe instantaneous temperature difference is influenced by the precedingtemperature difference.

In one refinement, the operating parameters of the internal combustionengine and/or the characteristics of the sheathed-element glow plug aremeasured for determining a stationary temperature difference duringstationary engine operation, and from this value the stationarytemperature difference is computed. Taking into account the operatingparameters of the internal combustion engine or the characteristics ofthe sheathed-element glow plug allows the temperature difference to bedetermined very accurately, since this temperature difference reflectsthe actual state of the internal combustion engine and of thesheathed-element glow plug, which are used in computing the temperaturedifference of the sheathed-element glow plug.

During stationary engine operation, the rotational speed of the internalcombustion engine and the injection quantity in the internal combustionengine are advantageously measured, from which a first stationarytemperature difference is computed. Since these operating parameters arealso detected for evaluating other situations of the internal combustionengine, no additional hardware is necessary to obtain these measureddata.

Alternatively, when air is static in the internal combustion engine thepower consumed by the sheathed-element glow plug which is necessary forreaching a certain predefined temperature is measured, and from thisvalue a second stationary temperature difference is computed. Theadvantage of this procedure is that the control unit of the motorvehicle, which is responsible for monitoring the glow temperature of thesheathed-element glow plug, is able to independently determine thissecond temperature difference without further auxiliary means. Thespecified glow parameters of the sheathed-element glow plug are thusalways maintained.

In another variant, the air mass in the internal combustion engineand/or the charge pressure of the air mass and the temperature of theair mass are measured, and from these values a third temperaturedifference is computed. In this case as well, the operating parametersnecessary for computing the third temperature difference are alreadyascertained for other purposes in the internal combustion engine, sothat additional provision of sensors or measuring equipment may bedispensed with.

In one refinement, the temperature difference, which is a function ofthe time function, is supplied to a system for controlling thetemperature of the sheathed-element glow plug, in which the temperaturevalue of the sheathed-element glow plug, which is represented by ameasured value, is ascertained as a function of the resistance of acurrent-conducting wire of the sheathed-element glow plug, thetemperature difference being added to the temperature value of thesheathed-element glow plug which is represented by the measured value,resulting in an actual controlled value. By using the time function fordetermining the temperature difference, the control system takes intoaccount the fact that the thermal flow from the glow wire inside thesheathed-element glow plug to the exterior of the sheathed-element glowplug requires a certain amount of time. This time offset is then takeninto account in the control, resulting in increased control quality.

Another refinement concerns a device for ascertaining a temperature of asheathed-element glow plug in an internal combustion engine, in which atemperature difference between the temperature of a sheathed-elementglow plug heater inside the sheathed-element glow plug and a temperatureat an arbitrary location on the sheathed-element glow plug is determinedas a function of at least one operating parameter of the internalcombustion engine. To allow the temperature difference between thetemperature of the sheathed-element glow plug at a location outside theheater and the temperature at the heater of the sheathed-element glowplug to be determined, means are present which determine the temperaturedifference as a function of a time function, and ascertain thetemperature of the sheathed-element glow plug from a temperature value,which represents a measured value, and from the temperature difference.This has the advantage that the temperature difference is determinedduring dynamic operation of the internal combustion engine, i.e., underthe variable operating conditions of the internal combustion engine, andchanging characteristics of the sheathed-element glow plug. This takesinto account in particular the fact that the surface of thesheathed-element glow plug is continuously cooled as a result of thefuel-air mixture flowing past the sheathed-element glow plug in thecombustion chamber.

In one embodiment, the sheathed-element glow plug protruding into thecombustion chamber of an internal combustion engine is connected to acontrol unit which determines the temperature difference, which dependson the time function, as a function of the operating parameters of theinternal combustion engine. In this regard, various control units may beused. For controlling the temperature of the sheathed-element glow plug,a glow time control unit is provided which cooperates with the enginecontrol unit of the internal combustion engine, the engine control unitproviding the glow time control unit with the measured operatingparameters for determining the temperature difference. In the glow timecontrol unit, the determined temperature difference is then taken intoaccount in controlling the sheathed-element glow plug temperature, inwhich this temperature difference is added to a temperature which isascertained as a function of the temperature-dependent resistance of thecurrent-conducting glow wire of the sheathed-element glow plug. Thetemperature difference between the heater and a point at the surface ofthe sheathed-element glow plug which is thus determined may be evaluatedfor temperature control or temperature regulation of thesheathed-element glow plug.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the configuration of a sheathed-elementglow plug in an internal combustion engine.

FIG. 2 shows a schematic flow chart for computing the temperaturedifference between the temperature of the heater and the temperature atan arbitrary point on the sheathed-element glow plug.

DETAILED DESCRIPTION OF THE INVENTION

Cold internal combustion engines, in particular diesel engines, atambient temperatures of <40° C. require starting assistance for ignitingthe fuel-air mixture which is introduced into the diesel engine. Glowsystems composed of sheathed-element glow plugs, a glow time controlunit, and glow software stored in an engine control unit are used asstarting assistance.

FIG. 1 shows such a glow system 1. A sheathed-element glow plug 2protrudes into combustion chamber 3 of diesel engine 4. Sheathed-elementglow plug 2 is on the one hand connected to glow time control unit 5,and on the other hand leads to a vehicle power supply voltage 6 whichactivates sheathed-element glow plug 2 with a nominal voltage of 11 V,for example. Glow time control unit 5 is connected to engine controlunit 7, which in turn leads to diesel engine 4.

For igniting the fuel-air mixture, sheathed-element glow plug 2 ispreheated in a “push phase,” which lasts 1 to 2 seconds, by applying anovervoltage. The electrical power thus supplied to sheathed-element glowplug 2 is converted into heat in a heater coil (not illustrated ingreater detail), which causes the temperature to abruptly increase atthe tip of sheathed-element glow plug 2. The heating power of the heatercoil is adapted to the requirements of the particular diesel engine 4,using electronic glow time control unit 5. The fuel-air mixture is ledpast the hot tip of sheathed-element glow plug 2 and is thus heated. Inassociation with the heating of intake air, the ignition temperature ofthe fuel-air mixture is reached during the compression stroke of dieselengine 4.

The determination of temperature difference ΔT of sheathed-element glowplug 2, which results from a difference of the heater temperature andthe temperature at a location at the surface of sheathed-element glowplug 2, is explained with reference to FIG. 2. Operating parameters ofdiesel engine 4 are measured in block 101 in FIG. 2. These operatingparameters include rotational speed n of diesel engine 4, injectionquantity q, air mass m_(air) of the air stream flowing into combustionchamber 3, charge pressure p, and temperature T_(air) of the inflowingair.

Static models for simulating temperature difference ΔT are then providedin block 102. In the present case, “static” means that these temperaturedifferences would result when diesel engine 4 operates in stationarymode. A distinction is made among three models:

In the first static model, temperature difference ΔT is determined as afunction of injection quantity q of the fuel and rotational speed n ofdiesel engine 4. For this purpose, a characteristic map is measured oncefor injection quantity q, rotational speed n, and changing temperaturedifference ΔT, with the aid of a measuring plug. Temperature differenceΔT is determined from this characteristic map stored in engine controlunit 7, based on the parameters rotational speed n and injectionquantity q measured in block 101.

The second static model for determining temperature difference ΔT isdetermined as a function of power P consumed by sheathed-element glowplug 2 for reaching a desired temperature T_(des). The followingexpression is valid:ΔT=a(T _(des))*exp(−b(T _(des))*(T _(des) /P))  (1)Ratio TP of temperature to powerTP=T _(des) /P  (2)is a measure of the efficiency of sheathed-element glow plug 2 or ameasure of the cooling of sheathed-element glow plug 2. The greater thepower P supplied to sheathed-element glow plug 2 to reach a desiredtemperature T_(des), the more intensely is sheathed-element glow plug 2cooled.

A third model for determining temperature difference ΔT is based on thedependency of the temperature difference of air mass m_(air) andtemperature T_(air) of the air, or on charge pressure p of the air andits temperature T_(air). It is assumed that charge pressure p increasesthe greater the quantity of air that is supplied to the combustionchamber. Temperature difference ΔT is computed as follows:ΔT=a*m _(air) +b+(c*T _(air) +d)  (3)orΔT=a·p+b+(c·T _(air) +d)  (4).

Coefficients a, b, c, d are based on measurements, and are determinedonce.

After the static models have been determined, a nonstationary model fortemperature difference ΔT is provided in block 103 as follows:ΔT _(nonstat) =ΔT _(old)*exp(−dt/τ)+ΔT·(1−exp((−dt/τ))  (5)where

-   ΔT=temperature difference of a static model determined in block 102-   ΔT_(old)=nonstationary temperature difference which was determined    in the preceding measuring cycle,-   dt=time interval of the measuring cycle,-   τ=time constant which indicates the thermal inertia of    sheathed-element glow plug 2 in the cylinder of diesel engine 4 in    which the sheathed-element glow plug is situated.

The exponential function in equation (5) indicates how stronglytemperature difference ΔT_(nonstat) to be instantaneously determined isinfluenced by the nonstationary value of temperature difference ΔT_(old)determined in the preceding measuring cycle.

In block 104, nonstationary temperature difference ΔT_(nonstat) is thenused in the control of the temperature of sheathed-element glow plug 2carried out by glow time control unit 3. Instantaneous temperature T ofthe sheathed-element glow plug is ascertained in that first, with theaid of current and voltage measurement at the glow wire ofsheathed-element glow plug 2, resistance R is ascertained, from whichtemperature T(R) is deduced. Alternatively, the temperature may beascertained by measuring the electrical power, or measuring the powerand the resistance. Temperature difference ΔT_(nonstat) determined inblock 103 is added to this temperature T(R), resulting in actualtemperature T.T=T(R)+ΔT _(nonstat)  (6)

In equation (6) it is taken into account that the thermal flow from theglow wire inside the sheathed-element glow plug to the surface of thesheathed-element glow plug requires a certain amount of time, which mustbe taken into account in the control as a time offset. Temperature Tthus ascertained is compared to the setpoint value of the temperature.The current supply to the sheathed-element glow plug is adjusted as afunction of the resulting difference in order to control thesheathed-element glow plug to the desired temperature.

1. A method for ascertaining a temperature of a sheathed-element glowplug in an internal combustion engine, comprising: determining atemperature difference between a temperature of a glow plug heaterinside the sheathed-element glow plug and a temperature at an arbitrarylocation on the sheathed-element glow plug as a function of at least oneof an operating parameter of the internal combustion engine and anoperating parameter of the sheathed-element glow plug, wherein thetemperature difference is determined as a function of a time function;and ascertaining the temperature of the sheathed-element glow plug froma temperature value represented by a measured value and from thetemperature difference determined as a function of the time function. 2.The method as recited in claim 1, wherein the time function isdetermined from a time constant ascertained for the sheathed-elementglow plug and stored.
 3. The method as recited in claim 2, wherein thetime function represents a measure of how strongly an instantaneouslydetermined temperature difference is influenced by a precedingtemperature difference.
 4. The method as recited in claim 3, wherein theinfluence on the instantaneously determined temperature difference bythe preceding temperature difference is represented by one of anexponential function or a power series.
 5. The method as recited inclaim 1, wherein the at least one of an operating parameters of theinternal combustion engine and an operating parameter of thesheathed-element glow plug is measured during stationary conditions ofthe internal combustion engine for determining a stationary temperaturedifference value, and wherein the stationary temperature differencevalue is used for determining the temperature difference determined as afunction of the time function.
 6. The method as recited in claim 5,wherein a rotational speed of the internal combustion engine and aninjection quantity are measured for determining the stationarytemperature difference value.
 7. The method as recited in claim 5,wherein a power quantity consumed by the sheathed-element glow plug andnecessary for reaching a predefined temperature is measured fordetermining the stationary temperature difference value.
 8. The methodas recited in claim 5, wherein at least one of an air mass, a chargepressure of the air mass, and a temperature of the air mass is measuredfor determining the stationary temperature difference value.
 9. Themethod as recited in claim 5, wherein the temperature differencedetermined as a function of the time function is supplied to a controlsystem for controlling the temperature of the sheathed-element glowplug, and wherein the control system determines a raw temperature valueof the sheathed-element glow plug as a function of a resistance of acurrent-conducting wire of the sheathed-element glow plug, and whereinthe control system generates an adapted temperature value of thesheathed-element glow plug by adding the temperature differencedetermined as a function of the time function to the raw temperaturevalue of the sheathed-element glow plug.
 10. A device for ascertaining atemperature of a sheathed-element glow plug in an internal combustionengine, comprising: a control system configured to perform thefollowing: determining a temperature difference between a temperature ofa glow plug heater inside the sheathed-element glow plug and atemperature at an arbitrary location on the sheathed-element glow plugas a function of at least one of an operating parameter of the internalcombustion engine and an operating parameter of the sheathed-elementglow plug, wherein the temperature difference is determined as afunction of a time function; and ascertaining the temperature of thesheathed-element glow plug from a temperature value represented by ameasured value and from the temperature difference determined as afunction of the time function.
 11. The device as recited in claim 10,wherein the sheathed-element glow plug protrudes into a combustionchamber of the internal combustion engine and is connected to thecontrol system.